Pulsed Doppler ultrasound systems are commonly used to measure and map the velocity of blood flow within human and animal bodies. Pulses of ultrasound energy are directed into the body along a path which intersects blood vessel or coronary chamber. Ultrasound energy from the pulse is backscattered from blood within the vessel or chamber and returns to a transducer where it is converted into an electrical signal. If the blood has a velocity component along the direction of propagation of the ultrasound waves, the frequency of the scattered echoes will be shifted, in relation to the frequency of the incident ultrasound energy. The Doppler shift which is thus induced in the echoes can be analyzed to yield a numeric estimate of blood velocity and/or to produce a map of blood velocity as a function of position within the body.
Doppler blood flow measuring systems are often included as an adjunct or accessory function to conventional ultrasound imaging systems which map acoustic impedance as a function of position within the body. Difficulties arise, however, because ultrasound signal requirements for Doppler blood flow measurement are substantially different than those required for high resolution ultrasound imaging. Conventional Doppler spectrum analysis for ultrasound blood flow measurement requires a narrow bandwidth signal, but narrow bandwidth inherently limits the range resolution which would otherwise be obtainable in an ultrasound imaging system. Short (and thus inherently wide band) pulses of ultrasound energy are used to maximize range resolution in imaging systems while long pulses with narrow bandwidth are used for Doppler measurement in order to obtain well defined spectral shifts and high signal-to-noise ratios.
One difficulty with conventional Doppler spectrum analysis arises from the interpretation of signals based on a Doppler shift model of scattering from moving blood cells. According to this model, Doppler frequency shift is proportional to the frequency of the incident ultrasound wave before it is scattered by the blood cells. A short pulse of ultrasound energy contains a wide spectrum of incident frequencies and results in a wide spectrum of scattered signals regardless of the velocity spread of the scattering blood cells. In addition, wide band filters (which are necessary in such systems) inherently result in a lower signal-to-noise ratio than would be possible with narrow band filters.
Prior art Doppler spectra are further subject to aliasing since they are periodic with a period equal to the pulse repetition frequency. Thus, from a prior art Doppler spectrum, one can only determine the velocity modulo (c/2) (f.sub.p /f.sub.O) where f.sub.p is the pulse rate. For example, if the RF ultrasound center frequency f.sub.O is 5.times.10.sup.6 hertz and the line rate is 5.times.10.sup.3 lines per second, a prior art one dimensional Doppler spectrum can only unambiguously determine velocities which are less than 0.75 meters per second (assuming the speed of sound is 1500 m/sec.).
"Ultrasonic M-mode RF Display Technique with Application to Flow Visualization" by P. M. Embree and W. T. Mayo, International Symposium on Pattern Recognition and Acoustical Imaging, Leonard A. Ferrari, Editor, Proc. SPIE 768, pp. 70-78 (1987) discusses a technique to display digitally sampled ultrasonic A-lines using false color. The RF samples of each backscattered A-lines using false color. The RF samples of each backscattered A-line are displayed vertically and consecutive A-lines are displayed side-by-side horizontally to form a two dimensional false color image. Fringe patterns in the two-dimensional image can be related to conventional Doppler processing and correlation processing concepts for fluid motion display. A rectangular section of the fringe pattern displayed in this way represents the two dimensional matrix of samples to be processed by the two dimensional Fourier technique which is the subject of this invention.